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Investigation of the Fate of Individual Sewage Disposal System Effluent in Turkey Creek Basin, Colorado Excerpted from CWRRI Completion Report No. 200, May 13, 2004 By Kathy Dano, Eileen Poeter, and Geoff Thyne Colorado School of Mines w ith rapid development and population growth in the Turkey Creek Basin (TCB) of Jefferson County, Colo- rado, the degradation of water quality has become a pressing issue. Residents of TCB are served by a fractured, crystal- line -rock- aquifer, typical of those in the western US that provide water to residential users through individual domestic wells and treat wastewater with individual sewage disposal systems (ISDSs). Comparison to basin -scale geochemical data from the 1970s and recent geochemical data from TCB reveals that specific conductivity (an indicator of water qual- ity) in the surface water has increased by a factor of 3.3 (see Table 1) over the past 30 years. Specific conductivity in the majority of the ground water has increased by a factor of only 1.2 over the same time period. However, specific conductiv- ity of ground water in localized areas has increased by a larger factor. This study investigates the role of ISDS effluent in the degradation of the basin's water quality by investigating the flow path and chemical evolution of ISDS effluent after it leaves the infiltration area of one individual sewage treatment system. Geophysical methods located the ISDS effluent plume of a s' 1 h Table 1. Comparison of water chemistry data from the 1970s (Hostra and Hall, 1975) and the late 1990s (Bossong et al., 2003). 1 14 tng a ome at the regohth- bedrock interface beneath and adjacent to an ISDS infiltration area. Shallow pie - someters were installed to measure hydraulic properties and monitor water level and quality. A water budget was calculated for the ISDS system to estimate the bedrock infil- tration rate. The home had a typical household pumpage of 644 L /day (170 gallons /day) of which —72 %, an average of 444L /day (123 gallons/ day), was dosed into the infiltration area from the septic tank. The low return rate is unexpected; an ongoing study is evaluating this finding. Under typical conditions, the efflu- ent infiltrates the fractured bedrock within 5 meters of the infiltration area, rather than migrating laterally through the regolith to the closest surface water, North Turkey Creek, which is 500 m away. During an unusually high spring runoff the plume migrated 50 to 100 m within the regolith before infiltrating the fractured bedrock. The chemical fingerprint of the ef- fluent is similar to the anthropogenic component required to account for the ground water quality decline as indicated by other studies. The chemical fingerprint of the effluent has a chemical signature similar to surface water near the mouth of the basin suQeestine that it contrih„tP.q to Ground Water 1975 1999 Parameter unit mean median # of samples mean # of median samples Specific Conductivity µS /cm 288 256 291 330 313 363 Calcium mg/L 40 34 40 39 36 269 Magnesium mg/L 10 8 40 9 8 270 Sodium mg/L 26 16 40 16 11 270 Potassium mg/L 2 2 259 2 2 142 Alkalinity mg/L 181 173 6 118 120 273 Sulfate mg/L 16 11 40 22 12 273 Chloride mg/L 9 4 291 25 7 269 Fluoride mg/L 1 0 40 1 1 242 Nitrogen (NO3 + NO2) mg/L 2 1 288 2 1 309 Surface Water 1975 1999 Parameter unit mean median # of samples mean median # of samples Specific Conductivity µS /cm 179 139 25 596 457 78 Calcium mg/L 16 141 24 72 42 56 Magnesium mg/L 4 3 24 16 10 56 Sodium mg/L 8 7 24 36 28 56 Potassium mg/L 2 1 24 3 3 34 Alkalinity mg/L 75 61 28 115 99 58 Sulfate mg/L 8 8 24 71 13 58 Chloride mg/L 9 6 79 65 58 Fluoride mg/L 1 0 J23 1 0 55 Nitrogen (NO3 + NO2) mg/L 0 0 1 0 47 Table 1. Comparison of water chemistry data from the 1970s (Hostra and Hall, 1975) and the late 1990s (Bossong et al., 2003). 1 14 tng a ome at the regohth- bedrock interface beneath and adjacent to an ISDS infiltration area. Shallow pie - someters were installed to measure hydraulic properties and monitor water level and quality. A water budget was calculated for the ISDS system to estimate the bedrock infil- tration rate. The home had a typical household pumpage of 644 L /day (170 gallons /day) of which —72 %, an average of 444L /day (123 gallons/ day), was dosed into the infiltration area from the septic tank. The low return rate is unexpected; an ongoing study is evaluating this finding. Under typical conditions, the efflu- ent infiltrates the fractured bedrock within 5 meters of the infiltration area, rather than migrating laterally through the regolith to the closest surface water, North Turkey Creek, which is 500 m away. During an unusually high spring runoff the plume migrated 50 to 100 m within the regolith before infiltrating the fractured bedrock. The chemical fingerprint of the ef- fluent is similar to the anthropogenic component required to account for the ground water quality decline as indicated by other studies. The chemical fingerprint of the effluent has a chemical signature similar to surface water near the mouth of the basin suQeestine that it contrih„tP.q to